1. Field of the Invention
The present invention relates to a composite substrate and a method for manufacturing a composite substrate.
2. Description of the Related Art
In general, composite substrates each including a base substrate and a substrate bonded thereto and composed of a group 13 nitride, such as gallium nitride, are used for manufacturing semiconductor devices such as blue LED, white LED, blue-violet semiconductor lasers, and power semiconductors. For example, Patent Literature 1 describes a method for manufacturing such a composite substrate, the method including bonding together a gallium nitride substrate and a base substrate through an oxide film. Specifically, first, the gallium nitride substrate and the base substrate composed of sapphire, silicon carbide, or alumina are prepared. Next, hydrogen ions are implanted into the gallium nitride substrate to form an ion-implanted layer, and an oxide film is formed on a surface of the base substrate. Then, the back surface of the gallium nitride substrate and the surface of the oxide film are polished, and then both substrates are bonded together with the oxide film formed therebetween. Then, the gallium nitride substrate is separated at the ion-implanted layer to produce a composite substrate. In this method, the oxide film is polished, and both substrates are bonded together through the oxide film, thereby achieving sufficient adhesive force.
In addition, in the method for manufacturing a composite substrate in Patent Literature 1, the ion-implanted layer is formed, and the gallium nitride substrate is separated so that a gallium nitride substrate remaining on the composite substrate has a desired thickness. The reason for this is that the separated gallium nitride can also be used for manufacturing another composite substrate, and thus expensive gallium nitride can be efficiently used as compared with a case where a desired thickness is achieved by polishing. The separation using ion implantation is also described in Patent Literature 2 and Non-Patent Literature 1. In particular, Patent Literature 2 describes a method for recovering crystallinity of a gallium nitride substrate which is decreased due to ion passage by ion implantation. Specifically, crystallinity can be recovered by annealing at 700° C. or more in an atmosphere of nitrogen-containing gas.
Also, Patent Literature 1 describes that in the method for manufacturing a composite substrate, the gallium nitride substrate and the base substrate are bonded through the oxide film and then heated to 900° C. or more in a non-oxidizing atmosphere to thin the oxide film by out diffusion. It is further described that an adverse effect of the low thermal conductivity of the oxide film on heat dissipation of the composite substrate can be minimized.